The present invention relates to a manufacturing method of a water-absorbent polymer (water-absorbent resin, high water-absorbent polymer, high water-absorbent resin, water-swelling/water-insoluble resin, water-absorbent agent, water-absorbent particles) by performing a polymerization of a hydrophilic monomer in a continuous manner, and more particularly relates to a continuous manufacturing method of a water-absorbent polymer which includes at least one of the steps of 1) subjecting a solution containing a hydrophilic monomer such as water-soluble ethylenically unsaturated monomer to a polymerization reaction after removing therefrom oxygen dissolved therein; 2) placing the solution containing a hydrophilic monomer to an inert gas deaerator, a mixer for mixing the solution with a polymerization initiator, a polymerization section, or the like; 3) detecting reaction temperatures of the polymerization reaction using a plurality of temperature detection means provided in the polymerization section (polymerization zone); and 4) transporting a water-containing gel (hydrogel) resulting from the polymerization of the hydrophilic monomer.
Water-absorbent polymers have been used in various applications: for example, absorbing materials for sanitary materials such as sanitary napkins, paper diapers (disposable diapers), etc.; drip absorbing materials; water retentive material for soil, or the like. These water-absorbent polymers are manufactured by performing a polymerization of a raw material of hydrophilic monomer (hereinafter may simply referred to as a monomer) in a batch manner or a continuous manner. The monomer is usually used in the polymerization reaction in a form of solution (hereinafter, referred to as a monomer solution). In this case; however, before carrying out the polymerization reaction, is performed the process of introducing an inert gas such as nitrogen gas, etc., into the monomer solution so as to remove therefrom oxygen dissolved therein.
The purpose of performing the above process is to suppress the adverse effects of oxygen (mainly oxygen dissolved in the solution) contained in the monomer solution on the polymerization reaction. By carrying out the polymerization reaction after removing oxygen contained in the monomer solution in the foregoing manner, the adverse effects of dissolved oxygen on the polymerization reaction can be suppressed, thereby permitting water-absorbent polymers having desirable properties to be manufactured at high yield.
In the case of polymerizing a monomer in a continuous manner, generally, an initiator of a polymerization reaction (hereinafter referred to as a polymerization initiator) is mixed with a monomer solution beforehand, and thus this continuous polymerization is performed by 1) mixing the monomer solution with a polymerization initiator in a mixing column (mixing vessel), and subsequently 2) supplying the monomer solution having the polymerization initiator mixed therewith on an upper surface of a transport belt polymerization device provided in a polymerization section via an inlet tube which constitutes a lower end part of the mixing column. In this way, the monomer in a form of a monomer solution can be supplied continuously, and a static polymerization (polymerization without stirring) can be performed on the belt in a continuous manner, thereby manufacturing water-absorbent polymers at high yield.
On the other hand, in the case of polymerizing a monomer in a batch manner, a polymerization initiator is mixed in a polymerization reaction container, and thus this batch-wise polymerization is performed by a) supplying a monomer solution in the polymerization reaction container such as a stainless tray or a kneader provided in the polymerization section, and subsequently mixing the monomer solution with a polymerization initiator in the polymerization reaction container. Alternately, the polymerization initiator can be mixed beforehand, and in this case, b) after the monomer solution is mixed with the polymerization initiator in the mixing column, the resulting mixture is supplied into the polymerization reaction container via the inlet tube which constitutes the lower end part of the mixing column, thereby polymerizing the monomer in a batch manner.
However, when manufacturing a water-absorbent polymer by performing a polymerization of a monomer, a temperature of the monomer (reaction temperature of the polymerization reaction) greatly vales due to heat generated in the polymerization reaction as will be explained in details below in reference to FIG. 8. Here, a monomer in the polymerization reaction indicates a mixture gel (reaction system) of the monomer and a water-absorbent polymer produced from the monomer.
As indicated by (A) in FIG. 8, at the beginning of the first half, reaction temperature increases gradually as the polymerization reaction proceeds smoothly, and then the reaction temperature starts increasing sharply towards a peak of the polymerization reaction (hereinafter referred to as a polymerization peak). For the second half of the polymerization reaction, i.e., the reaction after the polymerization peak, the reaction temperature drops sharply at the beginning, and starts decreasing gradually as time passes.
It is known that the described changes in reaction temperature in the polymerization reaction affect the properties of the resulting water-absorbent polymers or the yields thereof. For example, an excessive increase in reaction temperature (peak temperature) in the polymerization reaction causes the problems of: 1) an increase in ratio of unwanted secondary reaction; 2) an increase in soluble component in the water-absorbent polymer; 3) a reduction in absorbency with and without an applied pressure, and 4) a reduction in durability (gel stability against urine). On the other hand, an excessive reduction in reaction temperature in the second half of the polymerization reaction results in incomplete polymerization reaction, and this causes the problems of, for example, 1) an increase in an amount of monomer remaining in the water-absorbent polymer, 2) the resulting water-absorbent polymer is difficult to be handled due to an increase in viscosity, etc.
As a solution to the above problem, a device for use in producing water-absorbent polymers has been proposed by European Patent No. 922717A wherein the monomer is subjected to cooling for a predetermined time (reaction time from 0 to t in FIG. 8), and thereafter the monomer is subjected to heating for a predetermined time (reaction time after t in FIG. 8) when necessary. In the manufacturing device, a monomer to be polymerized is transported within the polymerization device by a transport belt, and first, a polymerization reaction of a monomer is performed while suppressing an excessive increase in the temperature of the monomer (reaction temperature) (i.e., under cooling for a predetermined time). Subsequently, a polymerization reaction is performed while suppressing an excessive decrease in temperature of the monomer (i.e., under heating for a predetermined time). As a result, variations in reaction temperature of the polymerization reaction as indicated by (B) in FIG. 8 can be achieved, and water-absorbent polymers having desirable properties can be produced at high yield.
The above water-absorbent polymers are initially produced in a form of a water-containing gel (water-containing gel-like crosslinked polymer). Then, the resulting water-containing gel is transported from the polymerization device to a gel granulator, for example, as disclosed in European Patent No. 925836A, to be granulated into granules of a predetermined size. The resulting granulated water-containing gel having a diameter of several tens cm square is then transported to, for example, a gel pulverizer such as meat chopper as disclosed in U.S. Pat. No. 5,275,773 to be pulverized into gel particles (one type of water-absorbent polymer) of a predetermined size (for example, a particle diameter of not more than 5 mm square). As necessary, the pulverized gel after being dried may be further subjected to grinding by a mill grinder to be water-absorbent particles or water-absorbent polymer particles (one type of water-absorbent polymer) of a size having a particle diameter in a range of from 10 xcexcm to 2000 xcexcm.
Transportation of the water-containing gel in and out from the polymerization device to the gel granulator, is performed by while being supported by a support face of the transport means (conveyer) such as an endless belt disposed between the polymerization device and the granulated gel (including inside the polymerization device or gel granulator).
(1) Polymerization of Monomer in the Exterior of Polymerization Device
Irrespectively of batch manner or continuous manner, when subjecting the monomer solution to the polymerization reaction, in some particular areas, such as mixing column, inlet tube, etc., retention or scattering of the monomer solution is likely to occur for their structures. For example, the mixing column or the inlet tube is generally structured so as to have a decreasing internal area from the top to the bottom (in this example, an area surrounded by a cross line between the inner surface of the mixing column or inlet tube and the horizontal plane), and for this structure, the retention of the monomer solution is liable to occur in the mixing column or inlet tube. Also, the monomer solution as being dispersed is liable to adhere to the outer surface of the inlet tube. Furthermore, such retention and dispersion of the monomer solution may cause a formation of a polymer from the monomer. This problem is likely to occur particularly in the case of adopting a monomer solution of high concentration (for example, not less than 10 percent by weight) and high viscosity.
Moreover, as described in Japanese Unexamined Patent Publication No. 126103/1986 (Tokukaisho 61-126103) (Publication Date: Jun. 13, 1986) (U.S. Pat. No. 4,656,232), a monomer solution containing an ethylenically unsaturated monomer is disturbed from being polymerized by oxygen dissolved in the monomer solution. Namely, oxygen is an excellent polymerization inhibitor for acrylic acid, and for this reason, generally, before starting the polymerization of the monomer solution, inert gas such as nitrogen gas is introduced into the solution for the purpose of removing therefrom dissolved oxygen. However, the monomer solution which has undergone removal of dissolved oxygen is very liable to be polymerized even in an absence of a polymerization initiator. Therefore, under conditions of an applied external factor such as stirring, light, etc., contaminated impurities, or highly concentrated monomer solution (for example, not less than several tens percent), etc., an undesirable polymerization reaction which causes the formation of the polymer in the exterior of the polymerization device is liable to take place.
The conventional method of supplying a monomer solution in a batch manner or a continuous manner, therefore has the following problems when polymerizing in the exterior of the polymerization device: 1) the polymer of the monomer adheres to and matures in a vicinity of a supply port (inner surface and outer surface) of the inlet tube for supplying the monomer solution to the polymerization section, or 2) the polymer adherers to and matures in the mixing column for mixing the monomer solution with the polymerization initiator.
Further, such polymer as adhered to and matured would disturb a smooth flow of the monomer solution, or 2) degrade properties of the resulting water-absorbent polymer as being contaminated in the monomer solution as foreign objects, and further raises the problem of wasting monomers as a raw material of water-absorbent polymers. These problems would be serious particularly in the case of continuously performing a polymerization of a monomer solution with which the polymerization initiator is mixed beforehand at industrial level.
(2) Deaeration From Monomer Solution
In the case of adopting the method of introducing inert gas in the monomer solution to remove therefrom oxygen dissolved therein, the resulting water-containing gel from a static polymerization has uncountable number of bubbles of inert gas having a particle diameter of several xcexcm to several hundreds of xcexcm dispersed therein. Such water-containing gel is formed probably because a polymerization reaction takes place with respect to the monomer solution containing a large amount of inert gas, and bubbles of the inert gas are dispersed in the resulting water-containing gel. This water-containing gel having dispersed therein bubbles of inert gas, etc., has the following problems.
In the case of grinding the water-containing gel after being dried in order to obtain water-absorbent particles (one type of water-absorbent polymer) having an average particle diameter in a range of from 200 xcexcm to 700 xcexcm, the portion around the spacing formed by the bubbles of inert gas, etc., is ground, and thus by-products of fine powders (for example, a particle size of not more than 150 xcexcm) or scaly fragments may be formed in a large amount besides the water-absorbent particles.
These by-products of fine powders, scaly fragments have high surface area to volume ratio, and thus have higher liquid absorbing rate as compared to the water-absorbent particles. Therefore, for example, in the case of uniformly mixing the surface treatment, these sub-products may absorb the surface treatment quickly which makes it difficult to uniformly apply the surface treatment onto the surface of water-absorbent particles. Furthermore, fine powders or scaly fragments having absorbed therein the surface treatment may agglomerate in a bulk, and a heating treatment to be applied to the water-absorbent particles after having applied thereto the surface treatment may not be performed desirably.
As a result of grinding after being dried, protrusions and recessions appear on the surface of the water-absorbent particles due to the bubbles of inert gas. The protrusions and recessions as appeared onto the surface are likely to be scraped, which may cause the formation of fine powders in other processes than grinding, or degrading of properties of the water-absorbent particles. More specifically, for example, when preparing water-absorbent particles (including transport process, etc.,) or processing water-absorbent particles (or water-containing gel) into water-absorbent material for use in paper diaper, 1) the surface which suffers from the process damage may be scraped, fine powders may be generated, or the surface treatment layer having applied thereto a surface treatment may be broken, or 2) properties of water-absorbent particles such as absorbency under pressure, permeability of swelled gel (water-absorbent particles having water absorbed therein) may be lowered. Furthermore, fine powders resulting from the grinding process or other processes may lower the operability. It is important to suppress an occurrence of such problem particularly when adopting the static polymerization in which a polymerization is likely to take place in a state where the monomer solution contains uncountable number of bubbles, such as the case of supplying a monomer solution onto the surface of the belt being driven, to continuously polymerize the monomer on the belt.
(3) Control of Polymerization Temperature
Changes in pattern of reaction temperatures in the polymerization reaction of the monomer are greatly affected even with slight changes in conditions, such as 1) a ratio of residual dissolved oxygen in the case of supplying a monomer in a form of aqueous solution (hereinafter referred to as an aqueous monomer solution) to a polymerization reaction, 2) thickness or initial temperature of the aqueous monomer solution fed on the transport belt, 3) set cooling or heating temperature of the monomer supplied to the polymerization reaction, 4) an amount of supply of a polymerization initiator, etc.
As described, the timing of the polymerization peak and the peak temperature at the polymerization peak are varied even with small changes in conditions. Therefore, even when adopting a manufacturing device provided with a heater for heating a reaction system for a predetermined period after a predetermined period of cooling treatment, water-absorbent polymers having desirable properties may not be obtained, or desirable productivity may not be obtained under stable conditions.
For example, in the case where an actual peak temperature is higher than expected, an excessive increase in the reaction temperature may not be fully suppressed even after the predetermined period of cooling treatment. Similarly, in the case where the timing of the polymerization peak appears at a later timing than expected, a predetermined period of the heating treatment may start being applied to the monomer before reaching the polymerization peak, and as indicated by (D) in FIG. 8, the reaction temperature increases excessively. Furthermore, in the case where an actual peak temperature is lower than expected, the polymerization reaction may not be performed to the sufficient level.
In an event of unexpected polymerization reaction, for example, as above-listed, the following problems would typically occur: 1) the water-containing gel is bursted or bumped out of the polymerization device in the middle of the polymerization reaction, 2) the water-containing gel adhering onto the transport means such as transport belt cannot be separated with ease, or 3) degrading of respective properties of a water-absorbent polymer as a final product occurs, or the like. Sometimes, however, it is difficult to determine from the appearance of the water-absorbent polymer whether or not degrading of respective properties occurs, i.e., whether or not an expected polymerization reaction is being performed. Therefore, in practice the respective properties of the water-absorbent polymer are confirmed by a physical property test to be performed with respect to final products after subjecting the polymerized water-containing gel to drying or (fine) grinding. For this reason, according to the described manufacturing method of the water-absorbent polymer using the described manufacturing device for water-absorbent polymers, such problem that a polymerization reaction may be continued for a long time without noticing an occurrence of abnormality, and a large amount of inferior water-absorbent polymer may be produced, resulting in the wasting of monomers as a raw material.
(4) Transportation of Water-Containing Gel
Generally, the resulting water-containing gel is viscous, and for this viscosity, the water-containing gel is liable to adhere to the support face of the transport means when transporting the water-containing gel in and out from the polymerization device to the gel grinder, and thus factors which hinder the water-containing gel from being transported smoothly such as the water-containing gel being entrapped, clogged, buckled, rolled in, etc., may occur. As a result, the water-containing gel may be cracked or cut as being extended. Further, 1) from the cracked or cut portion, the water-containing gel is liable to be rolled in between guide rolls or rotation rolls, or 2) the cracked or cut portion of the water-containing gel is liable to be clogged at the supply port of the gel grinder. Thus, a smooth transportation of the water-containing gel becomes more difficult.
Specifically, 1) when transporting the water-containing gel from the endless belt to the guide rolls, rotation rolls, a roller conveyer, etc., the water-containing gel adhering to the support face of the endless belt is difficult to be separated, which hinders the water-containing gel from being transported smoothly. 2) Also, in the case of transporting the water-containing gel on the guide rolls, rotation rolls, roller conveyer, etc., the water-containing gel is liable to adhere to the support face of the guide rolls or the rotation rolls, and the portion adhering to the support face may be rolled in a spacing between rolls, which hinders the water-containing gel from being transported smoothly. Particularly, in the case of performing a static polymerization of a water-soluble ethylenically unsaturated monomer, polymerization may be delayed (or uncompleted) in some parts, for some reasons. In this event, such parts possibly have extremely high viscosity, and thus would hinder the water-containing gel from being transported smoothly as being adhered to the support face of the transport means.
Furthermore, it is time consuming and troublesome to remove the entrapped or clogged portion of the water-containing gel. Particularly, in the case of performing a polymerization of a water-containing monomer (aqueous monomer solution) in a continuous manner, it may be necessary to terminate the entire operation of the manufacturing device including post-processing (drying, grinding, surface treatment, etc.).
An object of the present invention is to provide a method of manufacturing water-containing polymers having desirable properties in a continuous manner under stable conditions.
In order to achieve the above object, a continuous manufacturing method of a water-absorbent polymer by continuously performing a polymerization of a hydrophilic monomer using a solution containing a hydrophilic monomer, is characterized by including at least one of the following steps (A) to (D):
(A) introducing an inert gas into the solution in a continuous manner before subjecting the solution to a polymerization reaction, and subsequently removing the inert gas from the solution;
(B) supplying the solution to a polymerization process, and washing out the solution with water supplied before, at, or after a timing of supplying the solution to the polymerization process;
(C) detecting reaction temperatures of a polymerization reaction in non-contact manner at a plurality of points different distances away from a point where a solution is supplied; and
(D) supplying water to a water-containing gel and/or transport means when transporting the water-containing gel of the water-absorbent polymer obtained in or after the polymerization reaction.
According to the described method of the present invention, an occurrence of a trouble in a continuous manufacturing process of a water-containing gel of a water-absorbent polymer or necessity of terminating the manufacturing process can be avoided, and by overcoming deficiencies of the conventional method, water-absorbent polymers can be manufactured in a continuous manner under stable conditions.
Specifically, according to the method including the step (A), a polymerization reaction is performed with respect the monomer solution having undergone removal of inert gas introduced for the purpose of deaeration (removing oxygen). Therefore, as compared to the case of the conventional method, an amount of bubbles remaining in the solution can be reduced, thereby manufacturing a water-absorbent polymer which shows excellent handling efficiency and desirable properties.
According to the method including the step (B), water is supplied to an area (inner surface of a mixing column for mixing a polymerization initiator, etc.) where the solution contacts when supplying the solution containing a hydrophilic monomer. Therefore, unwanted adhesion and the retention of the solution or the adhesion and matured of the polymer of the hydrophilic monomer can be prevented. Namely, according to the method including the step (B), water-absorbent polymers can be manufactured in a continuous manner always under desirable conditions. Here, xe2x80x9cpolymerization processxe2x80x9d indicates a process of performing a crosslinking polymerization by improving a polymerization ratio of the monomer solution or water-containing gel at temperatures above room temperature, that is a process of maturing the water-containing gel. By carrying out this polymerization process, generally, a polymerization ratio of not less than 95 percent, or a not less than 99 percent or even not less than 99.9 percent can be achieved. Here, the polymerization process and (a part of) the drying process can be performed at the same time by evaporating in the polymerization reaction.
According to the method including the step (C), reaction temperatures of the polymerization reaction are detected (measured) over time, and a change pattern of the reaction temperatures can be monitored. As a result, whether or not an expected polymerization is being carried out can be determined in an early stage. Especially, by monitoring a change pattern of reaction temperatures around the peak temperature or in the second half of the polymerization reaction, reaction temperature(s) in the second half of the polymerization reaction, the peak temperature, or the timing of polymerization peak (peak timing) can be seen, and whether or not an expected polymerization is being carried out can be determined in an early stage under stable conditions.
According to the method including the step (D), such problem that the water-containing gel of the water-absorbent polymer adhering to the support face of the transport means can be prevented. Moreover, even the water-containing gel which has been adhered to the support face can be released therefrom. As a result, an occurrence of factors which hinder the water-containing gel from being transported smoothly can be prevented or suppressed. As a result, the problem of the water-containing gel being cracked or cut as being extended in the transportation process can be prevented, thereby permitting a continuous polymerization of water-absorbent polymers.